Modern semiconductor fabrication facilities (fabs) typically use a variety of different tools to perform various fabrication steps on a wafer, such as in the production of integrated circuits (ICs) on silicon wafers. For example, a fab may include tools for performing a variety of different processing and inspection steps such as, e.g., lithography, metrology, etching, ion implantation, deposition, etc. In order to efficiently transport work-in-process (WIP) among the various different tools to perform different fabrication steps on the WIP, a material handling system is generally employed in the fab. Material handling systems in current and next generation fabs typically include an automated material handling system (AMHS), and may also include some amount of manual transport of material by workers in the facility.
Automated material handling systems that are commonly employed in fabrication facilities include, e.g., overhead hoist transports (OHT), rail guided vehicles (RGV), automated guided vehicles (AGV), overhead shuttles (OHS), conveyor systems, and combinations thereof. Regardless of the particular type of AMHS used in the facility, the AMHS will often transport wafers to the various tools in lot containers, called cassettes or carriers, that each hold a plurality of wafers at a time. Commonly employed wafer carriers include mechanical interface (SMIF) pods and front opening unified pods (FOUPs), each of which may hold a plurality of wafers and enable access to individual wafers contained therein by a respective tool's material handler. With the emergence of 300 mm wafers, automated OHT systems which transfer the wafers in FOUPs have become increasingly common.
Each tool's station may generally include one or more material handlers for handling the WIP that is transported to it in lots via the fab AMHS. The material handler at a tool's station may generally include one or more load ports (LP) for loading and unloading FOUPs via the AMHS of the fab, as well as a robotic handling system for removing individual wafers from the FOUP and transferring them to the tool for a particular processing or measurement step.
The material handling system of the fab facility may also have one or more stockers for storing lots, such as, e.g., for temporarily storing wafers due to time deviations among the various tools, or for storage to facilitate transport of the stockers between various fabrication bays. Each stocker may have one or more load ports for loading and unloading FOUPs to and from the AMHS of the fab. The AMHS of a facility may also include a material control system (MCS) for controlling the flow WIP in the facility and issuing commands to the various transport and stocker modules in the AMHS.
By way of example and not by way of limitation, a typical AMHS in a semiconductor fab may utilize an OHT system that includes a track defining a route between various tools and/or stockers in the facility. The track may be located near a ceiling of the facility and have one or more computer-controlled OHT vehicles that travel on the track. Each vehicle may include a gripper for gripping a FOUP from a load port of a stocker or tool and a hoist mechanism that raises and lowers the gripper to raise and lower a FOUP. In a typical operation, the OHT vehicle may be positioned over a load port of a tool or stocker to lower the gripper and retrieve a FOUP from the load port. The hoist mechanism raises the FOUP and the OHT vehicle then transports the FOUP along the track to another load port of a tool or stocker in order to unload the FOUP for storage or a subsequent fabrication step. Each of these lot transfers may be performed according to a transport command received from the fab MCS.
There are currently available several different semiconductor fabrication facility layouts and material transportation architectures for moving wafers between the lithography processing stations and metrology or other inspection stations. Examples of metrology and/or inspection actions frequently performed in such configurations include, but are not limited to, overlay, critical dimension, focus, dose, film thickness, and macro and micro defect inspection. The rapid transportation of WIP from the lithography processing cell to the metrology station is particularly critical to lithographic semiconductor processing because of the possibility of lithography rework, wherein the WIP is returned to a lithography processing cell after metrology or inspection results, e.g., for resist removal and additional lithographic processing if the metrology or inspection results are outside of predetermined limits.
In FIG. 1, one such layout is depicted having a standalone configuration 100, in which a post-lithography metrology station 105a is a separate entity with its own wafer handler 110. A robot 120 within the cell 105b transfers individual wafers between FOUPs at one or more load ports 104 and a processing tool 106. The robot 120 transfers wafers between the cell 105b and the tool 106 via a slit valve 108. The metrology station 105a can be arbitrarily located in the fabrication facility but is often located close to the lithography processing cell 105b. FOUPs can be transferred between the metrology state 105a and cell 105b via the fab's AMHS. This configuration is advantageous in terms of flexibility and cost of ownership as it allows a single metrology station to serve a number of lithography processing cells, relying on the AMHS of the fabrication facility to convey material to and from the station. In this standalone configuration, wafers are generally transferred between the lithography processing cell 105b and the metrology station 105a in cassettes. However, the stand alone configuration 100 suffers from a major drawback of long and varying time to result, while the metrology system 105a waits for the AMHS of the facility to transfer a completed lot from the lithography cell 105b. 
In FIG. 2, another such layout is depicted having an integrated metrology configuration 200, in which the metrology station 205a is integrated with a lithography processing cell 205b. As in the configuration 100 shown in FIG. 1, wafers are transferred between FOUPs at one or more load ports 204 and the cell 205b by a robot 220. In this configuration, the robot 220 can directly transfer single wafers between a lithography tool 206 and the metrology station 205a for measurement. A crucial benefit of the integrated configuration 200 is that of time to result since wafers can be individually transferred into the metrology station 205a prior to completion of processing of the whole lot, thereby allowing feedback of results within lot processing time.
In FIG. 3, yet another such layout is depicted having an embedded metrology configuration 300, whereby the metrology station 305a is integrated into the exposure tool 325, and single wafers are transferred from the exposure tool 325 to the metrology station 305a for measurement immediately subsequent to exposure and prior to subsequent processing, such as develop or post exposure bake. This has additional benefits in terms of time to result but suffers from metrology challenges due to low image contrast prior to processing.
Both the integrated configurations and embedded metrology configurations suffer from cost of ownership disadvantages since the metrology station is dedicated to the lithography cell, resulting in substantial idle times for the metrology station during and between lot processing periods. Likewise, the standalone configuration suffers from long and varying time to result because it relies on the facility level AMHS to transfer completed lots form the lithography station.
It is within this context that aspects of the present disclosure arise.